The present invention relates to an improved gas bubbler system for a glass melt tank including hazardous substances, and preferably radioactive substances.
U.S. Pat. Nos. 5,188,649 and 5,340,372 to Macedo et al., which are hereby incorporated by reference in their entirety, teach the use of air bubbling by bubblers to increase the production rate of a glass melter. A gas bubbler is a device that delivers gas under the melt surface of a pool of glass in a glass melter. The gas rises to the melt surface in the melt pool. In this process the gas causes a forced convection current in the melt pool. At the orifice, through which the gas leaves the bubbler, the reaction between the gas, the molten glass, and the metal of the bubbler is maximized.
In the ""649 and ""372 patents, there is at least one horizontal, perforated INCONEL gas bubbler pipe, which is used to produce a curtain of gas bubbles between the electrodes in order to increase the specific electrical resistance of the melt. The gas bubbles also produce glass currents in the melt which distribute the feed materials on the melt surface, while delivering heat simultaneously from below, in order to achieve fast melting at relatively low temperatures.
However, the perforated pipes cannot be cooled sufficient to prevent corrosion, even by the oxygenated gas which flows therethrough. Therefore, the pipes are subjected to the aggressive effect of the glass and the oxygen at high temperatures, which results in a drastic reduction in the lifetime of the pipes, especially when the amount of oxygen is high. Furthermore, as a result of the relatively high location of the perforated pipes in the melter, the lower part of the melt, in which metallic components collect, is not sufficiently included in the circulation and oxidation of the melt, and contains elutable components.
For metals used in the gas bubbler such as INCONEL or other alloys of nickel and chromium, not only is there an attack similar to the attack at the melt line that tends to corrode metal faster than either above or below the melt line, but also there is an accelerated attack due to the fast moving liquid and gas phases. For INCONEL 690, the most commonly used metal for this type of furnace, the aperture of the orifice corrodes to an unacceptable level within two to four months.
In other patents which disclose the use of gas bubblers, the bubblers penetrate the bottom of the melter box (casing) which encase the waste glass melter. The bubblers are made of tubes of molybdenum disilicide. Due to the lack of redox control, a metal conducting phase can develop at the bottom of the melter that leads to melter failure at the bottom casing penetrations.
A second failure mechanism of the state of the art bubblers is that they develop clogging of the bubbler aperture. If a single air tube delivers air to many orifices, some will clog while others will excessively enlarge. Thus the bubbling will be uneven, reducing the effectiveness of the stirring in the melter, and thus its throughput.
The problem with waste melters is that economics often dictate that the glass composition has a high liquidus temperature. If the bottom of the melter is colder than the liquidus temperature, crystals will precipitate. In an attempt to prevent this precipitation, a bottom electrode is used to joule-heat the bottom of the melter. Even so, during short usage, the resistivity of the melter substantially changes indicating the precipitation of a conductive phase to the melter bottom.
In a first embodiment of the invention, it is desired to stir the contents of a glass melter and increase its output by use of a gas bubbler. A refractory metal insert is used to protect the bubbler from corrosion at the orifice of the bubbler through which gas is injected into the melt.
The use of a precious refractory metal insert at the bubbler orifice prevents the attack of molten glass on the bubbler. Even through precious metals may be much more expensive than INCONEL, the failure points of a bubbler are avoided by the use of a small amount of expensive metal, saving many times its value by reducing downtime of a very expensive operation.
The metal for insert is chosen from the base or platinum group metals. Preferably the precious metal from the platinum group is platinum or one of its alloys or one of ruthenium, rhodium, palladium, osmium and iridium. The base refractory metal is preferably chromium.
Another embodiment of the invention protects the orifices from clogging due to oxidation by preventing scales from being driven to the orifices by the gas supply. Clogging is avoided by internally coating or lining the high temperature region of INCONEL pipes to prevent oxidation. The lining can be a ceramic inner tube or a precious metal inner tube. Preferably the precious metal tube is made from platinum and its alloys.
Another embodiment of the invention includes the use of an AC powered, joule heated melter with at least three electrodes, one electrode in each side and one electrode in the bottom. The electrode preferably is made of a high chromium mixed alloy such as INCONEL.
The bottom electrode will have one or more orifices to bubble gas. The gas is preferably air, and most preferably an inert gas such as N2 or Ar. Preferably the orifices are protected by a precious metal insert. Also, preferably each orifice or group of orifices have a separate gas supply. Also, preferably each air passage is protected by a ceramic or precious metal liner.
A contributive failure mode occurs when multiple holes are tied to the same gas supply. Small variations in the pressure drop from hole to hole may induce the melt to enter the bubbling assembly and blocks some of the holes, making it much less useful as a glass melt stirrer. By the present invention, each hole is individually supplied with air from a level comparable to the melt level, preferably above the melt level, most preferably with individually controlled pressure.
The gas for the bottom electrode bubbling system is brought into the tank from the side of the tank. Alternatively, the electrode bubbling system extends from above the melt line behind the brick inner wall. Under either scenario, there is no tank bottom penetration in that, in the environment of use, namely hazardous materials including waste, and preferably radioactive waste, penetration of the bottom wall of the tank would be potentially dangerous.
Another failure mechanism of a hazardous waste, preferably radioactive waste glass tank, is associated with the support tubes which directly penetrate the melt pool from above. The vertical support tubes tend to fail close to the melt line.
There are normally two types of support tubes; one of the tubes acts as the air supply while the other acts as a thermal well. Both tubes tend to break or fail with equal frequency, indicating that the detrimental effect at the melt line is due to contact of the metal tubes with the molten glass agitated with bubbles and the plenum air above the melt line.
The protection of the support tubes can be achieved by adding a tube or protective sleeve to the outside of the support tubes at the melt line. This additional tube can be INCONEL (chromium-nickel alloy) or precious metal, preferably concentric tubes of metal and ceramic.
A preferred protection method is to bring the gas delivery tubes behind the ceramic inner wall of the melter. The tubes come through the side and out near the bottom of the melter or come through the side wall and up the ceramic assembly leaving the melter bottom at a higher level, preferably above the melt line.
The most vulnerable location to corrosive effects in a glass melt tank is the region where the gas (air) is first in contact with the melt. Referred to as the xe2x80x9cinverted melt linexe2x80x9d, the word xe2x80x9cinvertedxe2x80x9d is used to designate that the air/gas phase is below this melt line, while the molten phase is above this melt line. The present invention teaches the use of precious metal inserts to protect the metal of a gas bubbler line at the inverted melt line.
Accordingly, it is an object of the present invention to increase the lifetime of a bubbler assembly, preferably to be the same as that of the melter.
It is another object of the present invention to reduce the clogging of the orifices of the bubbler, as well as to reduce uneven flow of gas.
It is still yet another object of the present invention to have a well-stirred and hot furnace bottom that discourages the precipitation of a conductive phase.
These and other objects of the invention, as well as many of the intended advantages thereof, will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings.